This invention relates to cataphoretic depositions, and particularly to a method and apparatus in which a coating of insulating material of substantially uniform thickness can be obtained on and around an elongated body.
Cataphoretic deposition is a well-known technique for the deposition of insulating materials. It is widely used for the deposition of electron emissive material, e.g., emission carbonates. For example, one such method is disclosed in U.S. Pat. No. 3,020,224 entitled, "Method and Device for Applying Electron Emissive Coatings to Coating Supports of Indirectly Heated Thermionic Cathodes of Electrical Discharge Tubes," issued to H. H. Blank et al. on Feb. 6, 1962.
Conventional cataphoretic techniques are satisfactory for many purposes, e.g., fabricating electron filaments. However, these methods are incapable of creating coatings which are substantially uniform in thickness. This lack of uniformity is primarily due to two factors. Conventional depositions are taken from suspensions which are inhomogeneous due to settling of the particles in the suspension, and the electric field employed during the deposition is not radially symmetrical about the cylindrical body.
The nonuniformities caused by conventional cataphoretic depositions are particularly undesirable where a line source of thermionic electrons is desired. A line filament, i.e., a line source of thermionic electrons, would be useful as the cathode in a flat image display device. However, for a satisfactory display, it is desirable that the line filament operate under space charge limited emission conditions. Otherwise, the output current would be highly temperature dependent. In space charge limited emission, the distance between the filament and the effective anode through which the electrons are collected must be accurately maintained. For such an application, the conventional cataphoretically deposited filaments are unsatisfactory, resulting in unacceptable nonuniformity.